ByJulie Pace, Associated PressAugust 2, 2016

President Obama speaks during a news conference with Singapore's Prime Minister Lee Hsien Loong in the East Room of the White House on Tuesday. Mr. Obama spoke out against Republican presidential nominee Donald Trump.

WASHINGTON- — In a searing denouncement, President Obama slammed Donald Trump as "unfit" and "woefully unprepared" to serve in the White House on Tuesday. He challenged Republican lawmakers to drop their support for their party's nominee, declaring "There has to come a point at which you say enough."

The president's blistering critique of his potential successor came on the heels of Mr. Trump's criticism of an American Muslim family whose son, a captain in the US Army, was killed in Iraq. A growing number of GOP lawmakers have disavowed Trump's comments, but most of those who have endorsed him are sticking by that stance.

"If you are repeatedly having to say, in very strong terms, that what he has said is unacceptable, why are you still endorsing him?" Mr. Obama asked during a White House news conference. "What does this say about your party that this is your standard-bearer?"

The president said his opposition to Trump is about more than policy differences. He said that while he disagreed with his Republican opponents in the 2008 and 2012 elections, he never thought they were unfit to do the job.

Obama – who is enjoying heightened popularity in his eighth and final year in office – has made clear he plans to be an active player in the White House race, campaigning around the country for Democratic nominee Hillary Clinton. He and first lady Michelle Obama spoke at last week's Democratic convention in Philadelphia, where Khizr and Ghazala Khan also made an appearance.

Trump responded with a statement that summarized the points he makes in his stump speeches: "Obama-Clinton have single-handedly destabilized the Middle East ... released criminal aliens into our country who killed one innocent American after another ... produced the worst recovery since the Great Depression (and) shipped millions of our best jobs overseas."

Last week, Mr. Khan criticized Trump's call for a temporary ban on Muslims coming to the United States and challenged whether he had read the Constitution. Trump has questioned why Ghazala Khan did not speak, implying her religion prevented her from doing so, and has said he was "viciously attacked" by Khizr Khan.

Trump's comments have been criticized, especially by members of the military, as Anna Mulrine of The Christian Science Monitor reported.

Ryan McGill still grieves the loss of one of his best friends, a fellow pilot who was shot down during the Iraq war.

“I’m still heartbroken,” says the chief warrant officer in the United States Army Reserve, who flew Black Hawk helicopters in Iraq. “I talk to his family all the time just to check in, because it never goes away.”

It is with this friend in mind that Officer McGill says he will not consider voting for Donald Trump. For McGill, Mr. Trump’s response to a Muslim-American family who lost a son serving in Iraq was unprofessional enough to disqualify him.

“He wants to be the commander in chief, but to say that the sacrifices he’s made are on par with these families, or even greater than theirs – it’s horrible,” he says.

While troops potentially sacrifice their lives, families sacrifice even more, he adds. “They’re the ones who are left worrying and wondering every day whether we’re going to return,” he says. “When someone doesn’t, like Captain Khan, it’s heartbreaking.”

Prominent veterans from Sen. John McCain to the head of the Veterans of Foreign Wars have condemned Trump’s behavior toward Khan’s mother and father, who have spoken against Trump during and after the Democratic National Convention (DNC). Trump has since equated his sacrifices as a businessman with the Khans’ in losing their son, and he has also lashed out at the Khans for criticizing him.

Trump's criticism of the Khans is part of a familiar pattern for the Republican nominee: He can't let go of a perceived slight, no matter the potential damage to his presidential campaign or political reputation.

Trump spent the days after winning the Republican nomination criticizing a U.S. district court judge's Mexican heritage. The morning after accepting the Republican nomination at the party's convention, he re-opened months-old grievances with primary rival Ted Cruz.

Those who have worked with Trump say that in private meetings, he can often appear amenable to putting a controversy aside. But the businessman can quickly be drawn back in by an interview, especially if he believes he's already answered the question, or if he grows irritated by commentary on cable television.

Khizr Khan delivered an emotional address at last week's Democratic convention, with his wife standing by his side. The Pakistan-born Khan told the story of his son, US Army Capt. Humayun Khan, who was posthumously awarded a Bronze Star and Purple Heart after his death in 2004.

Trump's unwillingness to let the matter subside sparked outrage Monday from several Republicans.

Arizona Sen. John McCain, a former prisoner of war, said Trump did not have "unfettered license to defame those who are the best among us."

Rep. Mike Coffman, a vulnerable Republican in a competitive Colorado district, said he was "deeply offended when Donald Trump fails to honor the sacrifices of all of our brave soldiers who were lost in that war."

Missouri Sen. Roy Blunt said the Khans "deserve to be heard and respected."

"My advice to Donald Trump has been and will continue to be to focus on jobs and national security and stop responding to every criticism whether it's from a grieving family or Hillary Clinton," Blunt said in a statement.

But when asked about Khizr Khan on Fox News Channel's "Hannity," Trump responded, "His son died 12 years ago.... If I were president, his son wouldn't have died, because I wouldn't have been in the war, if I was president back then."

Trump didn't raise the controversy during a rally Monday in Ohio. His supporters in the crowd dismissed the matter.

But the real test for Trump isn't the opinion of the loyal supporters who attend his rallies. It's the broader general election audience, a far more diverse group still weighing his readiness for the White House.